# std::atomic_flag_wait, std::atomic_flag_wait_explicit

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
void atomic_flag_wait( const atomic_flag* object, bool old ) noexcept; | (1) | (since C++20)
void atomic_flag_wait( const volatile atomic_flag* object, bool old ) noexcept; | (2) | (since C++20)
void atomic_flag_wait_explicit( const atomic_flag* object, bool old, std::memory_order order ) noexcept; | (3) | (since C++20)
void atomic_flag_wait_explicit( const volatile atomic_flag* object, bool old, std::memory_order order ) noexcept; | (4) | (since C++20)

Performs atomic waiting operations.

Compares object->test(std::memory_order_seq_cst) or object->test(order) with old, and if they are equal then blocks until *object is notified by `std::atomic_flag::notify_one()` or `std::atomic_flag::notify_all()` (or the thread is unblocked spuriously). This is repeated until the values compare unequal.

1,2) The memory synchronization order is std::memory_order_seq_cst.
3,4) The memory synchronization order is order.
If order is one of std::memory_order_release and std::memory_order_acq_rel, the behavior is undefined.

These functions are guaranteed to return only if value has changed, even if underlying implementation unblocks spuriously.

### Parameters

object | - | pointer to the atomic flag to check and wait on
old | - | the value to check the atomic flag no longer contains
order | - | the memory synchronization ordering

### Return value

(none)

### Notes

This form of change-detection is often more efficient than simple polling or pure spinlocks.

Due to the ABA problem, transient changes from old to another value and back to old might be missed, and not unblock.

### Example

| This section is incomplete Reason: no example

### See also

notify_one (C++20) | notifies at least one thread waiting on the atomic object (public member function of `std::atomic_flag`)
notify_all (C++20) | notifies all threads blocked waiting on the atomic object (public member function of `std::atomic_flag`)
atomic_flag_notify_one (C++20) | notifies a thread blocked in atomic_flag_wait (function)
atomic_flag_notify_all (C++20) | notifies all threads blocked in atomic_flag_wait (function)

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